The present invention relates generally to prosthetics. More particularly, the invention relates to novel prosthetic systems incorporating unique superplastically-formed components, and to methods and equipment useful for fabricating the same.
The use of implanted prostheses in patient care is widespread. For example, dental endosteal implants are currently considered the standard of care for patients with missing teeth. As an illustration, a patient who is totally endentulous or is missing a large number of teeth may elect to have three to ten implants placed in either the maxilla or mandible, which become fixed or "osseointegrated" in the bone structure. These implants are often used to anchor a superstructure, which ultimately supports and distributes forces transmitted through an attached denture.
For purposes of illustration, a conventional method for preparing a superstructural prosthesis can be summarized as follows. First, an assembled implant will include a fixture, an abutment, and an abutment screw attaching the abutment to the fixture. Several such implants are placed into a cavity prepared in the jawbone structure, for example the mandible.
After the placed implants are osseointegrated (usually from 4 to 6 months after implantation), a corresponding number of gold or titanium cylinders are placed over the abutments. After careful work on investing and wax-up, a gold bar element is cast, which includes screw access holes to attach to the abutments. A denture prosthesis is fabricated including a removable denture base (usually fabricated from acrylic resin) to which an appropriate number of artificial teeth are processed and mounted. The denture prosthesis is ultimately connected to the implant-supported bar in an attachable/detachable manner.
There are several ways to attach the denture prosthesis ultimately to the implants. For instance, several artificial teeth mounted on the denture prosthesis may have central holes through which retaining screws are inserted, to extend through the underlying abutments and gold cylinders and thread into inner threaded portions of the connecting screws. Alternatively, a posterior side portion of the partial denture base can be provided with an extended base in which holes are made. Retaining screws are then placed through the holes into the inner threads of the connecting screws. In a further retention method, retaining clips may be attached to the undersurface (inner side) of the partial denture base. These clips are frictionally-engaged in a detachable manner to bar portions of the gold bar element.
The practical application of osseointegrated dental implant systems presents several challenges. Because natural teeth are fixed in the jaw bone through a periodontal membrane, an occlusal abnormality within biological limits will not cause any damage to the tooth structure. On the other hand, with an osseointegrated implant, the fixture lacks a periodontal membrane and anything less than a passive fit of a suprastructure to the implants may have an adverse influence directly on the jawbone structure. Moreover, such an abnormality will also cause a premature deterioration on the superstructure. Accordingly, the ideal superstructure will be prepared to provide a precise fit, taking into account mechanical environmental factors including stress load distribution and impact load condition.
As to dental implants systems overall, in general, it is believed that their long-term success is governed not only by good osseointegration of the endoseous implants to bone, but also by: (1) suitable biomechanics, materials and surface morphology; (2) correct indication and favorable anatomic conditions (bone and mucosa); (3) good operative technique; (4) patient cooperation (oral hygiene); and (5) adequate material selection and design/fabrication of the superstructure.
Osseointegrated dental implant systems can have overdentures that are either fixed or detachable. Detachable overdentures provide more versatility than fixed-overdentures. For instance, overdentures can be placed when there is an insufficient number of fixtures and/or when the fixtures are in a poor position for contouring a fixed prosthesis. In addition, still other anatomic, physiologic, esthetic, or oral hygiene limitations might dictate against an indication of a fixed prosthesis, requiring the use of a detachable overdenture.
As indicated above, a first component of an overall dental endosteal implant system is the implants themselves. Several important selection parameters for implant materials include tissue compatibility, corrosion resistance, fatigue strength, specific gravity, friction coefficient, and corrosive effect on other materials with higher potential. Given these parameters, titanium and several types of titanium-based alloys are preferred materials for implant fabrication.
Among the many different types of implants which have been developed, specifically, for promoting osseointegration, the Nobelbiocare implant system (which has merged with Sterioss implants), the IMZ (Intramobile Zylinder) implant system, (Implant, Innovation Incorp.) and the ITI (Internationale Team fur Implantologie) implant system are widely used. In the Nobelbiocare system, a fixture component made of unalloyed titanium is implanted into the bone structure. A component passing through the mucous membrane, known as an abutment, is connected to the fixture by abutment screws. Both the abutment and the abutment screws are also fabricated from unalloyed titanium. The upper surface of the abutment is connected to a gold cylinder, which forms a portion of the superstructure
The IMZ implant system is designed specifically for use with a patient's remaining natural dentition. Its three major components include an implant main body (unalloyed titanium substrate plasma spray-coated with titanium beads or hydroxyapatite powders), a transmucosal implant extension component which is implanted into the bone through the mucosa membrane, and an intramobile element or an intramobile connector. The intramobile element is normally manufactured using polyoxymethylene (POM) resins, having an excellent viscoelastic characteristics and mechanical properties. The intramobile connector is made of unalloyed titanium and is constructed in the form of a unified component of intramobile element and screw, the unified structure being connected to the POM disk. A plastic shock absorber, internally threaded to accept a prosthesis retention screw, screws into the implant to reduce stress on the bone surrounding the implant.
The ITI implant is characterized by a hollow cylinder structure. This implant is usually fabricated from unalloyed titanium, which is cold-worked to stabilize the crystalline structure and enhance its modulus of elasticity and toughness.
As indicated above, the overall system includes a separately-fabricated superstructure anchored to the implant(s). Various retention mechanisms have been employed for this anchorage, and the particular mechanism selected is important to the success of the system. Primary objectives of a superstructure retention mechanism are to make the superstructure detachable to make post-implantation observation/examination easier, to make the prosthesis adjustable (if necessary), and to provide for convenient extraction of the implants should a problem develop.
Retention bars are most commonly used for the retention mechanism, and are generally preferred because they provide strong retention force--typically providing at least 400 g of retention force to resist the attachment/detachment movement. Conventionally, retention bars are made of cast gold alloy (Type VI, containing 8.sup..about. 17 weight % of Ag, 9.sup..about. 16 weight % of Cu, up to 10 weight % of Pd, 0.2.sup..about. 8 weight % of Pt, and up to 3 weight % of Zn), although they are also sometimes cast from a silver-palladium alloy. The retention bar is firmly attached to the head portions of pre-placed implants. One mechanism for retaining the partial denture to the bar uses a plurality of bar riders, or clips, mounted on the bar. A corresponding number of clip retainers (or riders) are then placed on the undersurface of prosthesis base to cooperate with the bar riders.
Other retention methods include, for example, retention studs. These rely on a relatively small occlusal retention force, and thus provide less mechanical retention force than bar attachments. A resilient snap attachment is used with the IMZ implant, and includes a patrix which is screw-inserted into the implant main body and a cooperating matrix mounted inside the denture base. Other systems may use a Zagg or Zest attachment where limited space is a problem.
Similar to the selection of the material for implants, the selection of materials for the superstructure is very important. Several requirements for an optimal superstructure material include (1) excellent biocompatibility; (2) good electrochemical corrosion resistance; (3) light specific gravity; (4) compatibility to the elastic modulus of the receiving bone; (5) good physical strength and endurance; (6) excellent formability and dimensional stability; (7) less contamination of bacteria; and (8) good galvanic corrosion resistance. Three basic material groups have received serious consideration for use in superstructures, including metals, ceramics and polymeric resins/composites. Metals have been most promising because they best meet the aforementioned requirements. A variety of metals are available for fabricating a single crown to bone-anchored bridge. These include type IV hard gold alloy, gold-silver-palladium alloy, and commercially pure titanium and its alloys. However, Ni and Co elements are electrochemically unstable in-vivo. Co, Ni, Cu, Ag and V elements are considered toxic. Particularly, Ni element shows hypersensitivity and oncogenecity. Accordingly, gold alloys and titanium materials are preferred. It is generally believed that gold alloys exhibit excellent physical properties, castability, solderability, marginal fit, and relaxing the occlusal pressure.
Before any abrasive cleaning or finishing of the casting is done, the gold cylinders should be protected with protection caps or altered replicas. Negligence in this area may cause damage to the interface surfaces. This is one of the most critical steps in the prosthesis fabrication. Failure to protect the interface surfaces at this point will result in destroying the seats of the prosthesis.
There are several complications that occur in dental implant systems. Complications associated with the fixture (implant) are the most severe and usually result in loss of the fixture, including lack of or loss of osseointegration, fixture fracture, or mandibular fracture. If a terminal abutment is lost, any cantilevered extension on that side needs to be shortened if a fixed prosthesis is to be maintained. This shortening of the occlusal extension may result in loss of stability of an opposing complete denture. A less serious and infrequent complication associated with the fixture is that of dehiscence of the implant.
Loosening or breakage of the abutment screw hexagonal head can also occur. A screw that is fractured within the fixture is more difficult to remove because of limited visibility and access. Soft tissue inflammation and hyperplasia around the abutment can also occur. This finding is usually associated with a poor fit of the bar to the abutments which place stresses and strains on the integrated implant, which eventually lead to implant failure.
A loose abutment cylinder not only mechanically irritates the mucosa, but also allows an excessive accumulation of microorganisms at the interface between the abutment and implant fixtures. Ideally, the abutment cylinders must project 2-3 mm above the mucosal margin. If the interface between the gold cylinder and the abutment is not precise, then premature dehiscence will occur.
If the bar splint (which is used to accommodate attachments) is extensive, it may limit the potential for making a totally mucosa-born prosthesis, and distribution of loading forces between the mucosa and the splint is a factor in prosthesis assembly.
Complications associated with the denture prosthesis itself are the least threatening to the survival of the tissue-integrated prosthesis. These complications (breakage or dislodgement of acrylic resin teeth) are usually related to staining, wear, or breakage. The most common prosthesis problem is loosening or breakage of gold locking screws, with resultant loosening of the prosthesis.
There are some complications which can be significant, and more importantly, which indicate flaws associated with the design or fabrication of the implant-supported prosthesis. On occasion an overdenture or a fixed bone-anchored bridge framework will fracture. The most common fracture site is the area just distal to the terminal implant fixture (extension). Most such fractures are caused either by inadequate bulk in this region, or by voids or discrepancies formed during casting.
Prosthetic elements for implantation in other areas of the body also suffer several related disadvantages. Such elements desirably include components of maximal strength, minimal weight, and good resistance to electrochemical corrosion. Moreover, in many instances the prosthesis must include surfaces which conform and effectively cooperate with surfaces of other implanted structures, with anatomical features of the patient, or both.
In light of this background, there remain needs for novel prosthetic components that are lightweight, dimensionally accurate, and strong, and which exhibit superior corrosion resistance. Desirably, such components will be fabricated using accepted biomaterials and methods that are readily adapted to clinical practice. The present invention addresses these needs.